Root nodule bacteria are free-living soil bacteria, belonging to diverse genera within the Alphaproteobacteria and Betaproteobacteria, that have the capacity to form nitrogen-fixing symbioses with legumes. The symbiosis is specific and is governed by signaling molecules produced from both host and bacteria. Sequencing of several model RNB genomes has provided valuable insights into the genetic basis of symbiosis. However, the small number of sequenced RNB genomes available does not currently reflect the phylogenetic diversity of RNB, or the variety of mechanisms that lead to symbiosis in different legume hosts. This prevents a broad understanding of symbiotic interactions and the factors that govern the biogeography of host-microbe symbioses. Here, we outline a proposal to expand the number of sequenced RNB strains, which aims to capture this phylogenetic and biogeographic diversity. Through the Vavilov centers of diversity (Proposal ID: 231) and GEBA-RNB (Proposal ID: 882) projects we will sequence 107 RNB strains, isolated from diverse legume hosts in various geographic locations around the world. The nominated strains belong to nine of the 16 currently validly described RNB genera. They include 13 type strains, as well as elite inoculant strains of high commercial importance. These projects will strongly support systematic sequence-based studies of RNB and contribute to our understanding of the effects of biogeography on the evolution of different species of RNB, as well as the mechanisms that determine the specificity and effectiveness of nodulation and symbiotic nitrogen fixation by RNB with diverse legume hosts.